1. Field of the Invention
The present invention relates to an optical glass, and in particular relates to an optical glass having a high refractive index and a high dispersive power.
2. Description of the Related Art
With recent advent of digital cameras, instruments using optical systems have rapidly been integrated and functionalized, and the optical systems are increasingly required to be highly accurate, lighter and smaller. Thus, to realize this requirement, an optical design using an aspheric lens is becoming mainstream. Thus, a mold molding technique of directly forming an optical surface without requiring grinding and polishing steps has been more frequently employed in order to stably supply a large amount of the aspheric lens using a highly functional glass at low cost, and demands for optical glasses suitable for the mold molding and having high functionality (e.g., high refractive index, low dispersive power/high refractive index, high dispersive power) have increased year by year.
Precision press molding of glasses is a technique to obtain a glass molded article having a final product shape or a shape extremely close thereto and having an accurate surface, by using a molding tool having a cavity having a predetermined shape and molding a glass preform with pressure at high temperature. By employing the precision press molding, it is possible to produce a molded article having the desired shape, with high productivity. Thus, various optical glass parts such as spherical lenses, aspheric lenses and diffraction gratings are currently produced by precision press molding. In order to obtain the optical glass parts by precision press molding, it is obviously necessary to mold the glass preform with pressure at high temperature as described above. Thus, the molding tool used for the press is exposed to the high temperature and high pressure. Consequently, it is desirable to lower the glass transition temperature (Tg) of the glass preform as far as possible, from the standpoint of reducing damage of the molding tool itself and of the mold releasing film provided on an internal surface of the molding tool, due to the high temperature environment during the press molding. Glass having a high devitrification resistance is also strongly required for producing the glass preform.
Conventionally, optical glass having a high refractive index and a high dispersive power is typified by a composition containing lead oxide in a large amount, which has been used for precision mold press because the glass is highly stable and the glass transition temperature (Tg) is low. For example, the optical glass for the precision mold press, which contains lead oxide in a large amount, is disclosed in Patent Document 1.
However, the environment when the precision mold press molding is carried out is kept in a reductive atmosphere in order to prevent oxidation of a die. Thus, when lead oxide is contained as a glass component, reduced lead is precipitated from the glass surface and adheres to the die surface, thereby reducing the accuracy of the dimensions of the surface of the die. Also, lead oxide is harmful to the environment and has been desired not to be contained.
In response to such desire, various glasses containing B2O3, La2O3 and the like as essential components have been developed as optical glass for press molding, having a high refractive index and a high dispersive region and containing no lead oxide. These glasses are disclosed in, for example, Patent Documents 2, 3 and 4.
However, ZnO or alkali metal oxides such as Li2O or Na2O which are described to be useful for lowering the glass transition temperature is sparingly contained in the composition having a refractive index (nd) of 1.78 or more and an Abbe number (νd) of 16 or more, which is very useful for the optical design among the compositions of the glasses disclosed in these Patent Documents. Thus, they have high glass transition temperatures (Tg) and are less suitable for being mold pressed.
Patent Document 1: Japanese Unexamined Patent Application Publication No. Hei01-308843
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2000-128570
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2002-362939
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2005-239506